The ionic excitability mechanism of nerve and muscle membranes will be studied under voltage clamp to define in molecular and functional terms the properties of the various ionic channels. Frog nodes of Ranvier, muscle membranes, and neuromuscular junctions will be characterized by conventional transient analysis, fluctuation spectroscopy, and gating current methods. The kinetics of sodium channel gating and the flux mechanism of open neuromuscular junction channels will be analyzed in detail. Several drug receptors and the properties of potassium channels in muscle will also be studied.